The present invention relates to an improved process for producing highly pure aromatic carboxylic acid from an impure solid acid product, and, more particularly, to a process improvement whereby the solid acid product can be efficiently dissolved in a suitable solvent at relatively low temperatures despite the presence of solid lumps.
In typical commercial processes for producing highly pure aromatic carboxylic acids, and particularly isophthalic and terephthalic acid, a rather insoluble, impure acid is first produced and recovered from an oxidation process. In the oxidation process, which involves catalytic air oxidation of paraxylene in acetic acid solvent in the case of terephthalic acid, the impure acid, usually in the form of solid crystals, is conveyed from a dryer to a holding silo before further processing in a usually separate purification stage to remove impurities. In the purification stage of such a process, crude, i.e., impure, acid crystals are slurried in water, recycled mother liquor or other suitable solvent, and the resulting slurry is pumped through a series of preheaters to raise the slurry temperature and thereby dissolve the slurry particles. The resulting solution is then subjected to hydrogenation at elevated temperature typically in the range of 280.degree.-283.degree. C. under liquid phase conditions in the presence of a Group VIII noble metal hydrogenation catalyst. The purified acid is recovered by crystallizing the acid from the hydrogen treated solution. The principal impurities, which are p-toluic acid (m.p. 180.degree. C.; b.p. 275.degree. C.) derived from the compound 4-carboxybenzaldehyde (4-CBA) (m.p. 258.degree. C.; b.p. sublimes) and unidentified color bodies, along with some other organic components, such as benzoic acid (m.p. 122.5.degree. C.; b.p. 249.degree. C.) and some residual terephthalic acid, (in the case of terephthalic acid) remain dissolved in the solution.
Depending on the conditions under which the crude acid is dried, its moisture content, and the time during which the acid is held in the holding silo before entering the purification stage, significant lumping of the solid product can occur. These lumps can be as large as 10 cm in average diameter, and, in turn, they can be very difficult to process, can clog and even damage valves and conveying equipment, and can be very difficult to efficiently dissolve in water and other solvents, especially at relatively low initial temperatures, i.e., below about 100.degree. C., for further processing. The present invention provides a method for handling these lumps in the context of a process for producing highly pure aromatic carboxylic acid from a crude crystalline starting material.